Exhaust systems for internal combustion engines are conventionally built of components through which exhaust gas flows, on the whole, in all operating situations and together form the exhaust system. These components may be, in addition to one or more line sections, for example, one or more turbo chargers, one or more catalytic converters and/or one or more mufflers. Air correspondingly flows through exhaust systems for internal combustion engines in all operating situations, and exhaust systems usually have one or more filters, valves and compressors.
Exhaust systems and intake systems have recently started to be complemented by systems for actively influencing noise carried in the exhaust system or intake system, which can be attributed to the operation of an internal combustion engine. Such systems superimpose the noise, which is being carried in the exhaust system or intake system and is generated essentially by the internal combustion engine, with artificially generated sound waves, which muffle or change the noise being carried in the exhaust system or intake system. As a result, a sound released to the outside of the exhaust system or intake system shall fit the image of a particular manufacturer, appeal to customers and comply with legally required limit values.
This is achieved by at least one sound generator being provided, which is in fluidic connection with the exhaust system or intake system, and thus radiates sound into the interior of the exhaust system or intake system. This artificially generated primary sound and the secondary sound generated by the internal combustion engine are superimposed to one another and leave the exhaust system or the intake system together. Such systems may also be used for muffling. To achieve complete destructive interference of the waves of the noise being carried in the exhaust system or intake system, and of the sound generated by the sound generator, the sound waves originating from the loudspeaker must correspond in terms of amplitude and frequency to the sound waves being carried in the exhaust system or intake system, but have a phase shift of 180° relative to these. Even if the primary sound waves, which are being carried in the exhaust system or intake system and can be attributed to the operation of the internal combustion engine, and the secondary sound waves generated by the loudspeaker correspond to each other in terms of frequency and have a phase shift of 180° relative to each other, but the sound waves do not correspond to each other in terms of the amplitude, there will only be an attenuation of the noise emitted from the exhaust system or intake system.
An exhaust system with a system for actively influencing sound being carried in the exhaust system from the state of the art will be described below with reference to FIGS. 1 and 2:
An exhaust system 4 with a system 7 for actively influencing primary sound being carried in the exhaust system 4 has a sound generator 3 in the form of a sound-insulated housing, which contains a loudspeaker 2 and is connected to the exhaust system 4 in the area of a tail pipe 1 via a sound line. The tail pipe 1 has an orifice 8, which releases exhaust gas being carried in the exhaust system 4 and airborne sound being carried in the exhaust system 4 to the outside. An error microphone 5 is provided at the tail pipe 1. The error microphone 5 measures sound in the interior of the tail pipe 1. This measurement by means of the error microphone 5 takes place in a section located downstream of a mixing area in which the sound line opens into the exhaust system 4, and the fluidic connection is thus established between the exhaust system 4 and the sound generator 3. The term “downstream” is related here to the direction of flow of the exhaust gas in the tail pipe 1 of the exhaust system 4. The direction of flow of the exhaust gas is indicated by arrows in FIG. 2. Additional components of the exhaust system 4, for example, a catalytic converter and a muffler, may be provided (not shown) between the area of the fluidic connection of the exhaust system 4 and the sound generator 3, and the internal combustion engine 6. The loudspeaker 2 and the error microphone 5 are connected each to a control unit 9. Further, the control 9 is connected to an engine control unit 6′ of an internal combustion engine 6 via a CAN bus. The internal combustion engine 6 further has an intake system 6″. Based on sound measured by the error microphone 5 and operating parameters of the internal combustion engine 6, which are received via the CAN bus, the control 9 calculates for the loudspeaker 2 a secondary sound signal, which will generate a desired overall noise when superimposed to the primary sound being carried in the interior of the tail pipe 1 of the exhaust system 4, and emits this secondary sound at the loudspeaker 2. The control unit 9 may use, for example, a filtered-x least mean squares (FxLMS) algorithm and attempt to have a feedback signal/error signal measured by means of the error microphone go to zero by outputting secondary sound via the loudspeaker (in case of sound cancellation) or to go to a predefined threshold value (in case of sound influencing). Another bus system may also be used instead of a CAN bus.
The mode of operation of the control will be explained in more detail below with reference to FIGS. 3 through 5 based on the example of an active noise cancellation (ANC) control.
Many noises, which are generated by machines, for example, internal combustion engines, compressors or propellers, have periodic components. By monitoring the machine in question with a suitable sensor (e.g., tachometer), it is possible to provide a time-dependent input wave vector x(n), which has a dependence on the basic frequency and the predominant harmonics of the noise of the machine. For example, the exhaust gas back pressure, the mass flow of the exhaust gas, the temperature of the exhaust gas, etc., may be involved in this dependence and the input wave vector. Many machines generate noises of different basic frequencies; these are often called engine harmonics.
This time-dependent input wave vector x(n) has parameters, as is shown in FIG. 3, that have an influence on the signal generated by the noise source according to an unknown z-transformed transfer function P(z) of the noise source d(n) (the signal corresponding to generated primary noise to be superimposed). The time-dependent input wave vector x(n) is used by the control algorithm of a system for actively influencing sound (called “ANC core” in FIGS. 3, 4A, 4B and 6A) for generating a secondary sound used for the superimposition. A signal u(n) corresponds to the secondary sound used for the superimposition. The secondary sound when superimposed with the primary sound leads to a desired noise. The feedback signal e(n) corresponds to the desired noise. The signal u(n) corresponds (within the operating range) to the sound pressure of the secondary sound generated by a sound generator. The transfer function of the source Pz can be determined empirically.
The superimposition is symbolized in FIG. 3 by the summation sign Σ and takes place in the acoustic area (e.g., in an exhaust gas line). The feedback signal e(n) arising from the superimposition is detected, for example, by means of an error microphone, and returned to the control algorithm (ANC core) as a feedback signal.e(n)=d(n)−u(n).
The feedback signal e(n) thus corresponds to a sound pressure of the superimposed noise.
In FIG. 3, P(z) is the Z-transform of the transfer function of the noise source. This transfer function P(z) may depend, on the basic variable of the machine generating the noise (in this case a time-dependent input wave vector x(n) representing the speed of rotation), and on numerous physical parameters, for example, pressure, mass flow rate and temperature of the sound-carrying system. The transfer function of the noise source P(z) is not, as a rule, known exactly and is often determined empirically. For example, the engine can be operated at different values for different input parameters, the values of output parameters recorded, and input/output relationships determined.
It is known that the model of the ANC control shown in FIG. 3 has shortcomings, because the feedback signal e(n) contains components that cannot be attributed to the transfer function P(z) of the noise source. The feedback signal e(n) is returned to the control algorithm and is obtained from the superimposition of the signal d(n), which is generated by the noise source on the basis of the transfer function P(z), with the sound generated by the sound generator corresponding to the signal u(n).
The model of the ANC control is subsequently expanded by a second transfer function, or a sound system transfer function, of the sound generator S(z), as is shown in FIGS. 4A and 4B.
This second transfer function S(z) takes into account, on the one hand, shortcomings of the digital-analog (D/A) converters, filters, amplifiers, sound generators, etc., used in the electrical area, but also those of the path not yet taken into account in the acoustic area by the first transfer function P(z) from the location of the sound generation/sound superimposition to the location of an error microphone determining feedback signal e(n) and, finally, shortcomings of the error microphone, preamplifier, anti-aliasing filter and analog-digital (A/D) converter, etc., adjoining this in the electrical area.
In expanding the model from FIG. 3, the signal y(n) generated by the ANC core in the model of FIGS. 4A and 4B therefore takes into account the second transfer function S(z), which indicates the conversion of the signal y(n) generated by the ANC core into the u(n) signal. Here, u(n) corresponds to the (mathematically idealized) amplitude of the secondary sound generated by the sound generator.
The second transfer function S(z) takes into account the entire area from the output of the control (y(n)) to the feedback signal (e(n)) of the control.
When noises are generated by the noise source (i.e., the noise source is switched on), the second transfer function S(z) is obtained asS(z)=u(z)/y(z)and u(n) corresponds to the convolution of the signals s(n) and y(n)u(n)=conv[s(n),y(n)],wherein s(n) is the pulse response of the second transfer function S(z). e(z), y(z) and u(z) are the respective z-transforms of the signals e(n), y(n) and u(n).
FIG. 4B shows the model from FIG. 4A in more detail. As can be seen, the signal y(n) outputted by the ANC core is composed of two sinusoidal oscillations sin(ω0n), cos(ω0n), which are provided by a sine wave generator and are shifted by 90° relative to one another, and which were amplified before by different gain factors w1(n), w2(n) by means of two amplifiers in order to generate two signals y1(n), y2(n), which are shifted by 90° relative to one another with different amplitudes. The gain of the two amplifiers is correspondingly adapted dynamically by an adaptation circuit as a function of the feedback signal e(n).
If, for example, the ith engine harmonic EOi shall be cancelled for a certain speed of rotation RPM of the internal combustion engine, the basic frequency f0 to be canceled is obtained asf0=EO1·RPM/60.ω0=2 πf0.The adaptation circuit used to adapt the gain in FIG. 4B is operated with a clock frequency, which sets the clock frequency of the ANC core.
FIG. 5 schematically shows, the spectral frequency of, the amplitude (Magn) of the noise (noise(n)) over the frequency (Freq). Here, d(n) indicates the current sound pressure or loudness at the given basic frequency f0 in Pascal. ∥d(f)∥ shows the value of the amplitude at a defined time for harmonics.
The input wave vector x(n) of the ANC control is defined as follows (vectors are printed in bold):x(n)=[sin(ω0n), cos(ω0n)].
The paper “Active Noise Control: A tutorial review” by Sen M. Kuo and Dennis R. Morgan, published in the Proceedings of the IEEE, Vol. 87, No. 6, June 1999 is incorporated in its entirety by reference. This paper demonstrates that the ANC control minimizes the feedback signal e(n) after a build-up time. Reference is made to this paper in full extent and especially in respect to the narrowband feed forward control described there.y(n)=x(n)wT(n)=w(n)xT(n)=w1(n)sin(w0n)+w2(n)cos(ω0n).Here, xT(n) designates the transpose of the input wave vector x(n) and is obtained from a transposition of the columns and rows of the input wave vector x(n).
The vector w(n)=[w1(n), w2(n)] formed from the gain factors is called here the phasor vector of the ANC control.
As is shown in FIG. 4B, the gain of the sine waves is adapted by adaptation by means of the phasor vector w(n).w(n+1)=w(n)+μ·conv[s(n),x(n)]e(n),in which μ shows the rate of adaptation.
Since the transfer function S(z) is not known to the ANC core for each time. an estimate Ŝ(z) is used instead, so that the adaptation becomesw(n+1)=w(n)+μ·conv[{circumflex over (s)}(n),x(n)]e(n),wherein ŝ(n) is the pulse response of Ŝ(z).
The estimate of the transfer function Ŝ(z) of the sound generator is formed in the known manner. A comparison is made between the signal output from the sound generator with the signal input to the sound generator. Any difference is caused by the manipulation performed to the signal by the sound generator. This manipulation is termed the transfer function of the sound generator S(z). However, the true transfer function is difficult to obtain for complex systems. Therefore, the invention allows for the use of an estimate of the transfer function which essentially compares the signal output from the sound generator (20) with the signal input to the sound generator for multiple operating conditions to form what is termed the estimate of the transfer function Ŝ(z) and could also be termed an optimal or best available transfer function. This includes among other things the effect of a digital-to-analog (D/A) converter, reconstruction filter, power amplifier, loudspeaker, acoustic path from loudspeaker to error microphone, error microphone, preamplifier, antialiasing filter, and analog-to-digital (A/D) converter.
It was demonstrated in the state of the art that under the assumption that
a) the signal d(n) to be superimposed is a simple wave; and
b) the actuator used can provide an amplitude ∥u(n)∥≥∥d(n)∥,
it is possible to markedly reduce the average (AVG) of e(n):AVG[e(n)FINAL]˜0.
It is emphasized that the above explanations are only examples, and the present invention also includes other known possibilities for generating the signal y(n) outputted by the ANC core.
It is disadvantageous in prior-art systems for actively influencing sound that attempts are made, as a rule, to extensively or fully cancel a noise generated by the noise source. This leads to an extensively high load on the actuator being used, and the sound pressure level arising will therefore have a very irregular curve.